Medical catheters are used for the intravenous administration of therapeutic agents or nutrients. A catheter is inserted into the body through an incision into a moderately sized vein using an introducer and is threaded along the vein into a large vein such as the superior vena cava. Because of the large blood flow through that cavity, a concentrated infusate solution delivered through the catheter is rapidly diluted thus allowing the administration of a high concentration of therapeutic agent or nutrient.
Since the catheter is in-dwelling in the patient's body for a relatively long period, provision is made for preventing retrograde flow of blood or other body fluids into the catheter. Usually this is accomplished by providing a one-way valve at the distal end of the catheter. In many applications, the preferred type of valve is a so-called slit valve, an example of which is described in U.S. Pat. No. 4,327,722.
Basically, a slit valve is a one-way valve formed by one or more slits cut into the catheter wall near the distal end of the catheter. When the fluid pressure inside the catheter is greater than the fluid pressure outside the catheter by a selected amount, the opening defined by the slit expands. When the pressure outside the catheter is greater than the inside pressure, the valve opening will close and prevent fluid flow into the proximal end of the catheter. The pressure differential needed to open the valve may vary from catheter to catheter in accordance with the number of slits in the catheter, the length of the slits, the thickness of the catheter wall or the elasticity of the catheter wall.
In practice, the formation of a slit valve in the end of a catheter is a very exacting process. The edges of the slit must be shaped very precisely if the valve is to perform as intended, namely, to open only when the pressure inside the catheter exceeds the outside pressure by a predetermined amount. Also, in some applications, the desirable characteristics for the catheter per se are antagonistic to the proper operation of the valve in the catheter. For example, in a given case, the proper operation of the slit valve may call for a catheter wall which is thin and catheter wall material which is very soft and flexible. However, catheters made of such soft and flexible materials are difficult to insert into and thread along the patient's veins or other blood vessels to the selected infusion site. To facilitate the insertion of such catheters, ancillary wires or other stiffening means have been incorporated into the catheters; see U.S. Pat. No. 4,559,046, for example. Obviously the inclusion of such stiffening devices increases the overall cost of the catheter. Moreover, those devices must be removed after the catheter is inserted thereby complicating the insertion process as a whole.
Also, in some cases it is desirable to withdraw blood or other fluids from the body through an implanted catheter. While it is usually possible to cause a slit valve to open by creating a lower fluid pressure inside the catheter than outside, in practice it is generally not possible to withdraw fluids through a conventional catheter with a slit valve because the catheter, being soft and thin-walled tends to collapse and close off if the internal fluid pressure is lowered.
Finally, many catheters in use today are connected to implanted vascular access ports. In such use, the catheter must be connected securely and reliably to the access port. Making such a connection is difficult if soft thin-walled catheter tubing must be used.